Clear air turbulence detector

ABSTRACT

This disclosure describes an apparatus for warning the pilot of an aircraft of a region of clear air turbulence. A multi-channel radiometric sensor mounted on the aircraft detects both the ambient temperature of the air and any temperature anomaly that is present along the forward flight path. In those cases where temperature anomalies are associated with the presence of a clear air turbulence region, the invention provides means for remotely sensing these temperature anomalies through the application of a unique radiometric technique. By the detection of difference temperatures between a minimum of two channels, the invention provides a means for indicating the existence of a temperature anomaly indicative of clear air turbulence region. In addition, the distance from the aircraft to the anomalous temperature region is directly determined from the output indication of the radiometric sensor channels. The distance between the clear air turbulence region and the aircraft is determined by utilization of at least two observing frequencies which have known absorption coefficients of different values.

United States Haroules et al.

" atent [72] Inventors: George G. Haroules, Lexington; Wilfred E. Brown,111, Acton; Harold I. Ewen, Weston; Arthur E. Lilley, Belmont; Ralph D.Kodis, Newton, all of Mass.

[73] Assignee: The United States of America as represented by theAdministrator of the National Aeronautics and Space Administration [22]Filed: Nov. 19, 1969 [21] Appl. No.: 877,990

[52] U.S. Cl. ..343/100 ME, 73/355, 325/363,

[| I Int. Cl ..G0lw 1/00 [58] Field ofSearch ..343/100 ME; 73/355;325/363 [56] References Cited UNITED STATES PATENTS 3,056,958 /1962Anderson ..343/100 ME UX 3,028,596 4/1962 McGillem et a1. ..343/100 ME3,380,055 4/l968 Fow et a]. .343/l00 ME 3,465,339 9/1969 Marner..343/100 ME [57] ABSTRACT This disclosure describes an apparatus forwarning the pilot of an aircraft of a region of clear air turbulence. Amulti-channel radiometric sensor mounted on the aircraft detects boththe ambient temperature of the air and any temperature anomaly that ispresent along the forward flight path. In those cases where temperatureanomalies are associated with the presence of a clear air turbulenceregion, the invention provides means for remotely sensing thesetemperature anomalies through the application of a unique radiometrictechnique. By the detection of difference temperatures between a minimumof two channels, the invention provides a means for indicating theexistence of a temperature anomaly indicative of clear air turbulenceregion. In addition, the distance from the aircraft to the anomaloustemperature region is directly determined from the output indication ofthe radiometric sensor channels. The distance between the clear airturbulence region and the aircraft is determined by utilization of atleast two observing frequencies which have known absorption coefficientsof different values.

7 Claims, 6 Drawing Figures I I 5| p owes I 0.0. I W22 I 46 47 JENVELOPE oTo L 49 nsrecron M sYNc nzrecron REF 6 n ENVELOPE AUD SYNC 49I0 uerscron AMP DETECTOR 5| I 0.0. I memo I L mres I IPoweR|, 1 AT 57 'JCIRCUIT INDICATOR PATENTEDmza m2 sum 1 or 4 too so so 70 so 5040 30 20l0 0 INVENTORS RANGE IN km K WS Wm UWWmm N mm L m 08. K H in w GE G A dw e w m a mm w 6W m r A E a Y B 2 G F.

PATENTi-Inmma I972 3. 665.467

George G. Horoules Wilfred E. Brownm Hamid LEwen Arihur Edward LilleyaBY Ralph D. Kodis 5 W ATTORNEY;

PATENTEDmza I972 3,665,467

sum u 0F 4 i A, i

OVEN N2 INVENTORS George G, Horoules Wilfred E. Brown III Harold l. EwenArthur E. Lilley & 6? Ralph D. Kodis A ORNEYS CLEAR AIR TURBULENCEDETECTOR ORIGIN OF THE INVENTION The invention described herein may bemanufactured and used by or for the United Stages Government forgovemmental purposes without payment of any royalty thereon or thereforfBACKGROUND OF THE INVENTION Clear air turbulence (CAT) regions are ahazard to modern aircraft because the effects of moving through a CATregion may result in severe structural damage to the aircraft. In fact,passing through a CAT region may even result in the loss of theaircraft. CAT can be generated by atmospheric processes that aremarkedly different from each other in terms of their physical anddynamic characteristics. Some of these atmospheric processes arepredictable and, consequently, CAT caused by them is detectable. Forexample, CAT associated with mountain waves is predictable from the dataobtained and disseminated by weather stations. Similarly, CAT due tostrong vertical convection currents usually found in the proximity oflarge cumulus clouds is detectable using weather radar. However, otherCAT regions are not easily detectable. For example, the CAT which occursin jet stream frontal regions is not easily detectable.

It will be appreciated from the foregoing description of the effects ofCAT that it is desirable to provide a system that detects CAT regionsand warns an aircraft pilot of their existence so that the pilot cantake evasive action.

Therefore, it is an object of this invention to provide a new andimproved apparatus for detecting clear air turbulence regions bydetection of their related temperature anomaly.

It is also an object of this invention to provide an apparatus forwarning a pilot of a clear air turbulence region that is suitable foruse on aircraft traveling faster than the speed of sound.

It is another object of this invention to provide an apparatus forwarning a pilot of a clear air turbulence region that is low in powerconsumption, low in complexity and low in weight, making it suitable foruse on modern aircraft.

It is still another object of this invention to provide an apparatus formeasuring the range to and temperature anomaly associated with a clearair turbulence region, interpreting the measured data, and warning thepilot of the occurrence of the clear air turbulence region in sufficienttime for the pilot to avoid the clear air turbulence region.

SUMMARY OF THE INVENTION In accordance with a principle of thisinvention, an apparatus for detecting atmospheric temperature anomaliesassociated with regions of clear air turbulence is provided. Theapparatus comprises a multifrequency radiometric receiver, an antenna,and a signal processing system. The radiometric sensor by means of itsantenna detects a temperature dif ference in the form of a microwavesignal when the sensor system is mounted in an aircraft and the aircraftis moving in a forward direction toward the temperature anomaly. Thesystem uses two or more different frequencies that are associated withthe different absorption coefficients of gases in the atmosphere. Thetechnique will also detect density anomalies when there is notemperature anomaly. Utilizing the characteristic of molecular resonancein terms of absorption coefficients over small bandwidths, allows thesensor to simultaneously observe the thermal radiation of the atmosphereover a large dynamic range of absorption coefficients determined by theappropriate selection of frequency. The use of atmospheric oxygen assuch a gas is advantageous because of its well defined molecularresonances and line structure. That is, at a particular flight altitudea desired absorption coefficient in the range from 0.5 db/km to severaldb/km can be chosen by merely selecting the frequency of observation.

The radiometric sensor looking forward along the flight path detectstemperatures consisting of two components the ambient temperature, To,along the flight path, plus a difference temperature, AT,, relative tothe ambient temperature. The values of AT, observed at frequencies withdifferent absorption coefficients differ in magnitude, depending on thehorizontal range to the CAT region. The lower the absorptioncoefficient, the greater the range at which a AT is detected. The higherthe absorption coefficient, the shorter the range. As the aircraftapproaches the CAT region, the frequency of observation corresponding tothe highest absorption coefficient ultimately provides a larger value ofAT than all other frequencies of observation, even though the frequencycorresponding to the lowest absorption coefficient provides the firstdetection of a temperature difference AT relative to the ambient.

An appropriate indicating means is connected to the output of themultifrequency sensor to provide the pilot with an indication of thetime before encountering the region of clear air turbulence. Theindicating means also indicates the distance from the aircraft to theregion of clear air turbulence.

In accordance with a still further principle of this invention, themultifrequency radiometric sensor is balanced by noise injection so thatonly temperature differences are sensed when it is being used todetermine the range to the clear air turbulence. That is, the radiometerchannels are adapted to sense only temperature differences at thefrequencies of operation of each of the channels, not temperatures on anabsolute scale.

' It will be appreciated by those skilled in the art and others that theinvention is a rather uncomplicated apparatus for detecting regions ofatmospheric temperature anomalies along the flight path of an aircraft.A multifrequency radiometer having a balanced input is utilized to sensethese temperature anomalies along the flight path of the aircraft. Theuse of a multifrequency system results in a sensor that senses theambient temperature along the flight path, as well as anomaloustemperature regions forward of the aircraft along the flight path. Inaddition, the power requirements of a radiometer fall within the powercapabilities of a modern aircraft power plant. Further, a radiometerbeam is not harmful to the eyes of pilots of other aircraft as is alaser beam, for example.

BRIEF DESCRIPTION OF THE DRAWINGS The foregoing objects and any of theattendant advantages of this invention will become more readilyappreciated as the same becomes better understood by reference to thefollowing detailed description when taken in conjunction with the ac--companying drawings wherein:

FIG. 1 is a pictorial diagram utilized to describe the ranging theory ofoperation of the invention;

FIG. 2 is a graphical diagram of the normalized response of aradiometric sensor to a step temperature anomaly as a function of range;

FIG. 3 is a graphical diagram of the normalized response of aradiometric sensor to a ramp temperature anomaly as a function of range;

FIG. 4 is a graphical diagram of the normalized response of aradiometric sensor to an exponential temperature anomaly as a functionof range;

FIG. 5 is a functional block diagram of a preferred embodiment of theinvention; and

FIG. 6 is a functional block diagram of a modified absolute radiometricmode or temperature difference mode.

DESCRIPTION OF THE PREFERRED EMBODIMENT Prior to describing thepreferred embodiment of the invention, the following description of thetheory of the operation of the ranging aspects of the invention ispresented.

To illustrate the range capability of a radiometric temperature probingsystem, first consider one frequency of observation (one channel) inwhich the atmospheric attenuation coefficient determined by theobserving frequency is constant throughout the ray path. The antennatemperature for this condition is:

T,,=(1 l/L)T,, 1 where T the antenna temperature;

T the ambient temperature along the horizontal flight path at the flightaltitude; and

L the total attenuation of the atmosphere along the flight path Thetotal attenuation L in the above equation must be expressed in anumerical value. It is conventional to refer to the attenuation of theatmosphere in the units of db/km. For example, if the attenuation is onedb per kilometer, then in a path km long, forward of the aircraft, thevalue of L is 20 db (numerical value 100); and, the value, T sensed bythe radiometer from this 20 km path, is 99 percent of the value of theambient temperature at the flight altitude. It is apparent from thissimple example that if there is no temperature anomaly along the flightpath, then the temperature sensed by the radiometer is the ambienttemperature along the flight path.

Next, consider the case in which a temperature anomaly occurs over asmall range increment forward of the aircraft. This condition is shownin simplified form in FIG. 1. The radiometer is shown on the farleft-hand side of the diagram. The range to the CAT region is S,. Theextent of the CAT region is 8 -8,. The column of air beyond the CATregion extends to infinity (for all practical purposes in thisanalysis).

For the first step in the analysis, assume that the temperature isconstant along the entire flight path; i.e., that a CAT temperatureanomaly does not exist. However, for ease of analysis, the individualtemperature contributions from each of the three regions is hereinafterdeveloped.

L, the total attenuation over the path 5,;

L the total attenuation over the path 8 -8,; and

L the total attenuation beyond the CAT region.

Then, the contribution to the antenna temperature from the region S, is:

obtained directly from Equation (1). The temperature radiated by the CATregion (S S,) is:

However, this radiated temperature is attenuated as it passes throughthe path S,; hence, the temperature of the CAT region observed at thesensor is:

In a similar manner, the temperature radiated by the region beyond theCAT measured at the far boundary of the CAT re- Since this region isconsiderably greater in extent than the range increments S, and S thetemperature radiated by this region in the direction of the sensormeasured at the far boundary of the CAT region will be equal to theambient, To. That is:

T To 6 However, the temperature contribution from this region, measuredat the sensor, also suffers the attenuation of the CAT region (S S,) andthe region 8,; hence, the contribution at the sensor is:

s s and s 7 Therefore, the temperature at the sensor consists of threecomponents: T s an s Surpmiggygan si nplifying Equations 2, 4, and 7provides the following result:

As anticipated, the antenna temperature equals the ambient temperaturealong the flight path consistent with the assumed condition that therewas no temperature anomaly in the region S, 8,.

Now assume a temperature anomaly ATo is in the region S, S,. This changein the analysis is accommodated by noting that Ts, now takes the form:

1 To ATo L2) L. (9) which can be rewritten in the form:

a a -i 2 i a Q Since the second term in Equation 10 represents the onlydifference in the summation, relative to the prior condition in whichthe temperature was considered to be constant throughout the horizontalflight path, then it is apparent that the antenna temperature now sensedby the radiometer takes the form:

1 ATo T,lT0+( -L2)-IT (11) Equation 1 1 can be rewritten in the form:

1 1 vT,,T0-l-' l (1"L1)AT0 1 1 r-izxl vtwl 13 where:

Therefore, the observed differential temperature, relative to theambient temperature is a function of:

a. the temperature anomaly in the CAT region, ATo;

b. the extent in range 5,), of the CAT region and the atmosphericattenuation associated with the CAT region as reflected by the value ofL and,

c. the range to the CAT region S, from the radiometer and theatmospheric attenuation associated with this path as reflected by thevalue of L,.

It should be noted at this point that the values of L, and L are notonly range dependent, but also dependent on the absorption coefficient(a in db/km) of the medium which is in turn dependent on the frequencyof observation. Reference is made to an article entitled, The MicrowaveSpectrum of Oxygen in the Earths Atmosphere," by M. L. Meeks and A. E.Lilley, in Volume 68, No. 6 Edition of the Journal of GeophysicalResearch, dated Mar. 15, 1963, for a discussion of the dependency of theatmospheric absorption coefficient on frequency of observation atfrequencies near 60,000 GHZ.

The following mathematical example more clearly demonstrates the abilityof a radiometer to detect a temperature anomaly in range, as aconsequence of the difference observed in the values of AT as a functionof the frequency of observation; i.e., absorption coefficient.

Consider the following conditions:

a. ATo 10K b. out 0.1 db/dm attenuation at one frequency c. a3 l .0db/km attenuation at the second frequency d. Extent ofthe CAT region (SS,) 10 km Using the foregoing values, the temperature radiated by theCAT region at the two frequencies of observation can be computed. Theradiated temperature is given by the term in brackets in Equation 14.

These values are listed in Table l.

TABLE] adb/km (5 -8 km I (db) L (no) ll/L ATa] aA=0.l 1.0 1.26 2K aB=l.0 10 10.0 10.0 9K

The value of AT associated with the CAT anomaly as observed at thesensor can now be computed as a function of range to the CAT region bymerely introducing the appropriate attenuation of 2K and 9K signals overthe range S remembering, of course, that the attenuation value in eithercase in db/km must be applied. These values are listed in From Table IIit is apparent that at a range of 10km, the frequency with the lowestvalue of a (i.e., 04A 0.1db/km) provides a temperature differenceresponse of l.61(, while the frequency with the high value of a (i.e.,118) provides a response of only 0.9K. At 50 km, however, the responsein the high a channel is undetectable, while the channel with the low orvalue provides a response of 0.65K.

Equation 12 can be rewritten in the form:

The right-hand side of Equation 15 is a function of the a value, therange to the CAT region, and the extent of the CAT region. The left-handside of Equation 15 is in the form of a normalized temperature response;i.e., the numerator is the difference between the observed antennatemperature and the ambient temperature along the flight path whichvaries as a function of range. The denominator is the actualthermometric temperature anomaly associated with the CAT region.

Rewriting Equation 15 in prior notation form results in:

0 T To/ATo AT lATo 16 In Table II the values of AT for two assumedvalues of a a temperature difierence (ATO) of 10K, and a CAT extent of10 km was computed. A graphical plot of AT /ATo for the same condition(a step change) is shown in FIG. 2. To convert the normalized value oftemperature to a measured value for AT the vertical scale only need bemultiplied by 10K. As shown in FIG. 2, the differential temperature, ata range of 100 km for the a 0.1 db/km channel, is 0.2K; at 50 km,0.65I(; and at 10 km, l.6K. The corresponding response for the a 1.0db/km channel as a function of range is negligible until the aircraftapproaches to within approximately 20 km of the CAT region. At 10 km,the observed temperature difference rises sharply to 0.9K. FIG. 2 alsoincludes other frequencies of observation which provide absorptioncoefficients of 0.2, 0.5, and 1.5 db/km for comparison purposes.

In the prior example, the temperature anomaly associated with CAT wasassumed to be a step-function; i.e., an abrupt change in temperatureretaining a constant value throughout the CAT region. For comparisonpurposes, a graphical plot, similar to FIG. 2 for a ramp temperatureincrease is illustrated in FIG. 3, and an exponential temperatureincrease is illustrated in FIG. 4. It is of interest to note that thegeneral form of the response is essentially the same independent of theform of the temperature anomaly.

Turning now to a description of the preferred embodiment of theinvention illustrated in FIG. 5, an antenna 11, which may be a scanningtype, a multibeam type or a single beam type, is connected to anorthogonal mode transducer 13, which allows the signal received by theantenna to be separated into two polarizations. One polarization isseparately applied to each of a pair of channels, illustrated in FIG. 5.More specifically, the upper portion of FIG. 5 is devoted to one channeland the lower portion of FIG. 5 is devoted to the second channel. Morefrequencies or channels may be added if desired to provide a finer rangegranulation. The region between the two channels in FIG. 5 is devoted tocomponents that are common to both channels.

Each frequency channel of the multifrequency radio-metric sensor sharesthe antenna 11 and comprises a RF processing section and a signalprocessing section. The channels also share a common output indicatorsystem which is controlled by a common timing and reset circuit.

Each radiometric RF processing section comprises: first and second sidecouplers l5 and 17; a modulator 19, including its driver unit 21; anisolator 25; a mixer 27; a preamplifier 29; first and second attenuators31 and 33; a controlled oven reference (heat sink) 35; a klystron orsolid state local oscillator 37; and, first and second power supplies 39and 41 for the klystron and the modulator driver units 21. Each signalprocessing section comprises: a third power supply 43; an IF amplifier45; an envelope detector 46; an audio amplifier 47; a synchronousdetector 49; an integrator 51, and a common reference generator 53. Anoutput indicator circuit 55, a AT circuit 57 and a clock and resetcircuit 59 are common to all frequency channels.

The signal outputs from the orthogonal mode transducer are connected toone of the inputs of the first coupler 15 of each channel. The firstcoupler 15 at the front of the modulator 19 allows the addition of noisefrom a first noise generator 61. This noise is fed to the side inputs ofthe first coupler 15 via the first attenuator 31. The first attenuator31 is used to adjust the amount of added noise such that the inputsignal and added noise equals the input signal appearing at thereference input port of the modulator 19.

The output of the modulator 19 is connected through the isolator 25 toone input of the mixer 27. The klystron 37, which acts as a localoscillator, is tunable and connected so as to receive its input DC powerfrom the power supply 41. The klystron is also mounted so as to applyits heat to the heat sink 35. The RF output of the klystron 37 isconnected through the second attenuator 33 to the second input of themixer 27. The output of the mixer 27 is connected through thepreamplifier 29 to the input of the IF amplifier 45. The output of theIF amplifier is connected through the envelope detector 46 to the inputof the audio amplifier 47. The power supply 43 is connected to thepreamplifier 29, the IF amplifier 45 and the audio amplifier 47 toprovide power to those amplifiers. The output from the audio amplifier47 is connected to one input of the synchronous detector 47. Thesynchronous detector is switched at the same frequency as the modulator19.

Also illustrated in FIG. 5 is a calibration section, a referencegenerating section, and a control section, all of which are common toboth channels. The calibration section comprises: a second noisegenerator power supply 75 and a second noise generator exciter 73.

The reference noise generating section comprises a reference generatorpower supply and monitor 67, a reference control 69, and a referencetemperature dual oven load 71. The output of the reference generatorpower supply and monitor 67 is connected through the reference control69 to supply power to the reference temperature dual oven load 71. Thedual oven load 71 is connected to inputs of the second couplers 17. Thesecond couplers have their outputs connected to the modulators 19. Thepurpose of this section is to monitor the gain of each channel of theradiometric sensor so as to detect a temperature difference bysubtraction of the two independent receiver channel measurements afternormalization of their individual gains by the previously describedprocedure.

The control section comprises: a power supply 77; and, a program source79. The power supply 77 is connected to provide power to the referencegenerator 53 and to the program source 79. The output of the programsource is connected to the noise generator power control 75 of thesecond noise generating section. The reference generator 53 generatestwo reference signals. One reference signal is applied to the modulatorand the synchronous detector of the first channel and the otherreference signal is applied to the modulator and the synchronousdetector of the second channel.

The output of the synchronous detector 49 of each channel is connectedthrough the DC amplifier and integrator 51 of that channel to separatorinputs of the ATcircuit 57 forming a part of an indicator network. Theindicator network also includes the clock and reset circuit 59 and theindicator 55. The clock and reset circuit 59 generates a clock signalwhich is applied to the AT circuit 57 and a reset signal that is appliedto the indicator 55. The output from the AT circuit 57 is also appliedto the indicator 55.

Turning now to a detailed description of the operation of the overallembodiment of the invention illustrated in FIG. 5, each modulator actsas a single-pole, double-throw switch. More specifically, each modulator19 provides an amplitude modulated noise signal which is proportional tothe temperature difference between the noise powers presented to themodulators input port from the orthogonal mode transducer 13 and to themodulators comparison port. The reference generator 53 provides themodulation control signal while the power supply 39 provides power tothe modulator 19. The RF noise power with the modulated component isamplified at the RF frequency of the channel by the mixer and RFamplifier 27. The RF signal frequency of each channel is determined bythe frequency of the signal generated by the klystron and preamplifierfrequency. The signal frequency is further amplified by the preamplifier29 and the IF amplifier 45. Thereafter, the signal is detected in theenvelope detector 46. Finally, the envelope signal or modulatedcomponent (which is in the audio frequency range) is amplified by theaudio amplifier 47.

The audio amplified signal is compared in the synchronous detector 49with the reference generator signal. The output from the synchronousdetector has a voltage level that is proportional to the inputtemperature difference. The output from the synchronous detector isamplified by the DC amplifier and integrator 51 in order to obtainthe'average value of the amplitude modulated signal. The outputs fromthe DC amplifier and integrators of the two channels are compared in theAT circuit 57 and the output from the AT circuit 57 is applied to theindicator to provide an indication of any temperature difference which,as heretofore described, is an indication of a CAT region.

The operation of the dual channel sensor illustrated in FIG. andgenerally described above is based upon a modification of the absolutepower measurement concept described in U.S. Pat. Application Ser. No.686,248 for Method and Apparatus for Providing an Absolute PowerMeasurement Capability, by George Haroules, et al, filed Nov. 28, 1967,now U.S. Pat. No. 3,564,420. The invention described in that patentapplication is a passive circuit that is connected at the input of arelative power measurement radiometer to provide an absolute powermeasurement capability. This invention varies the circuit described inthe foregoing patent application by only introducing a first noisegenerator 61, a first attenuator 31, and a first coupler 15 prior to themodulator 19. The first noise generator 61 is excited by the first noisegenerator exciter 65 and its power supply 63. The purpose of thismodification is to inject an amount of noise which when added to theinput signal such that the sum appearing at the signal port of themodulator 19 balances the signal appearing at the reference port of themodulator 19. The reference port is the port coupled through the couplerto the reference noise generator 53. A functional block diagram of theRF balance circuit of the invention is illustrated in FIG. 6. Thiscircuit is the key feature of the invention in that two or more channelsare modified such that a normalized power subtraction is achieved.

With the foregoing modification, the radiometer is balanced when theantenna views the ambient temperature along the flight path. In themodified absolute mode it will measure a temperature difference ATo froman ambient temperature To.

It should be noted that an important teaching of the invention isillustrated by the dashed lines of FIG. 5 surrounding varioussub-sections of the invention. That is, the dashed lines illustrate thatall of the RF components are at the same temperature. By maintaining allof the RF components at the same temperature, the summation of termsmaking up transmission signal path losses of prior art devices are madetime invarient and hance allow the calibration and balance of the systemto be independent of time.

The program 79 controls the programming of the generation of noise fromthe second noise generator 73 by controlling the noise generator powersupply 75. The couplers couple the noise sources and the signals (eitherthe signal from the orthogonal mode transducer 13 or the referencesignal from the reference 71) to the modulator 19. The ATcircuit 57compares the signals representing the change in ambient temperature sothat an indication of the occurrence of a CAT region can be provided tothe indicator. The indicator 55 is adapted to indicate the distance fromthe aircraft to a temperature anomaly associated with a clear airturbulence region. The AT circuit 57 indicates the occurrence ofdistance to a clear air turbulence region by comparing the output of theradiometric channels.

The signal amplitude received from the anomalous temperature regionincreases as the sensor approaches the region of the phenomenon. Theamount of the temperature anomaly that fills the main beam (fillingfactor) determines the magnitude of the received signal together withthe differential temperature and range extent of the CAT region.

It will be appreciated from the foregoing description that the inventionis a dual channel radiometric sensor suitable for sensing and detectingatmospheric temperature anomalies associated with clear air turbulenceregions in accordance with the theory presented. It will also beappreciated that various modifications can be made within the scope ofthe teachings of the invention. For example, it may be desirable to havemore than two channels so that simultaneous measurements at variousranges can be accommodated along the forward path of the aircraft duringflight. In this manner, various distance indications of clear airturbulence regions can be provided. Consequently, alert, critical orother indicating warnings can be provided to the pilot so that he cantake the appropriate evasive action to avoid CAT regions. Hence, theinvention can be practiced otherwise than as specifically describedherein.

What is claimed is:

l. A clear air turbulence detector comprising:

a multifrequency radiometer means having a plurality of channels forgenerating radiometric probes at predetermined frequencies and forgenerating output signals for each channel, said output signals beingrelated to the anomalous temperature characteristics of the regionsbeing probed;

indicating means connected to said multifrequency radiometer means forsensing the output signals from said channels and for displaying saidoutput signals in a predetermined manner;

an antenna connected to said multifrequency radiometer means;

said multifrequency radiometer means includes two channels, each of saidchannels including:

a radiometer processing section connected to said antenna for processingthe signals received by said channels so as to generate output signalsfor differential power comparison;

a signal processing section connected to said radiometric processingsection for processing signals from said radiometric processing sectionand for generating a processed signal related to the range to saidregions of atmospheric temperature anomalies being probed; and

an orthogonal mode transducer connected to said antenna to receivesignals from said antenna and to said radiometric processing sections toapply the received signals to said radiometric processing section.

2. A clear air turbulence detector for probing the atmosphere to detectatmospheric temperature anomalies associated with clear air turbulenceregions, including a plurality of radiometric channels, wherein each ofsaid channels comprises:

an RF processing means adapted to receive RF signals related to thetemperature and altitude of regions being probed;

a signal processing means connected to said RF processing means forprocessing the signals processed by said RF processing means so as toobtain a signal including information about the range to anomaloustemperature regions along the forward flight path of high performanceaircraft;

indicating means connected to said signal processing means forinterpreting the signals generated by said signal processing means andfor displaying information about the occurrence of clear air turbulenceregions; and

a calibration means connected to said RF processing means for balancingthe output of said RF processing means when a clear air turbulenceregion is not being probed.

3. A clear air turbulence detector as claimed in claim 2, wherein saidcalibration means includes a noise generator.

4. A clear air turbulence detector as claimed in claim 3, including areference noise generating means adapted to generate a reference noisesignal, said reference noise generating means being connected to saidsignal processing means and said RF processing means for applying areference noise signal to said signal processing means and said RFprocessing means.

5. A clear air turbulence detector as claimed in claim 4, including acontrol means connected to said reference noise generating means forcontrolling the noise generated by said reference noise generatingmeans.

6. A clear air turbulence detector as claimed in claim 5, including anantenna means connected to the inputs of said RF processing means forreceiving RF signals and applying the received RF signals to said RFprocessing means.

7. A clear air turbulence detector as claimed in claim 6, includingsignal splitting means connected to said antenna means and to said RFprocessing means for splitting the RF signal received by said antennameans and applying said signals to said RF processing means.

1. A clear air turbulence detector comprising: a multifrequencyradiometer means having a plurality of channels for generatingradiometric probes at predetermined frequencies and for generatingoutput signals for each channel, said output signals being related tothe anomalous temperature characteristics of the regions being probed;indicating means connected to said multifrequency radiometer means forsensing the output signals from said channels and for displaying saidoutput signals in a predetermined manner; an antenna connected to saidmultifrequency radiometer means; said multifrequency radiometer meansincludes two channels, each of said channels including: a radiometerprocessing section connected to said antenna for processing the signalsreceived by said channels so as to generate output signals fordifferential power comparison; a signal processing section connected tosaid radiometric processing section for processing signals from saidradiometric processing section and for generating a processed signalrelated to the range to said regions of atmospheric temperatureanomalies being probed; and an orthogonal mode transducer connected tosaid antenna to receive signals from said antenna and to saidradiometric processing sections to apply the received signals to saidradiometric processing section.
 2. A clear air turbulence detector forprobing the atmosphere to detect atmospheric temperature anomaliesassociated with clear air turbulence regions, including a plurality ofradiometric channels, wherein each of said channels comprises: an RFprocessing means adapted to receive RF signals related to thetemperature and altitude of regions being probed; a signal processingmeans connected to said RF processing means for processing the signalsprocessed by said RF processing means so as to obtain a signal includinginformation about the range to anomalous temperature regions along theforward flight path of high performance aircraft; indicating meansconnected to said signal processing means for interpreting the signalsgenerated by said signal processing means and for displaying informationabout the occurrence of clear air turbulence regions; and a calibrationmeans connected to said RF processing means for balancing the output ofsaid RF processing means when a clear air turbulence region is not beingprobed.
 3. A clear air turbulence detector as claimed in claim 2,wherein said calibration means includes a noise generator.
 4. A clearair turbulence detector as claimed in claim 3, including a referencenoise generating means adapted to generAte a reference noise signal,said reference noise generating means being connected to said signalprocessing means and said RF processing means for applying a referencenoise signal to said signal processing means and said RF processingmeans.
 5. A clear air turbulence detector as claimed in claim 4,including a control means connected to said reference noise generatingmeans for controlling the noise generated by said reference noisegenerating means.
 6. A clear air turbulence detector as claimed in claim5, including an antenna means connected to the inputs of said RFprocessing means for receiving RF signals and applying the received RFsignals to said RF processing means.
 7. A clear air turbulence detectoras claimed in claim 6, including signal splitting means connected tosaid antenna means and to said RF processing means for splitting the RFsignal received by said antenna means and applying said signals to saidRF processing means.